1. Field of the Invention
The invention is related generally to the use of multi-component resistivity measurements for determination of properties of earth formations. In particular, the present invention discusses a method of reducing the non-formation parasite effects in multi-component resistivity measurements.
2. Background of the Art
Electromagnetic induction resistivity well logging instruments are well known in the art. Electromagnetic induction resistivity well logging instruments are used to determine the electrical conductivity, and its converse, resistivity, of earth formations penetrated by a borehole. Formation conductivity has been determined based on results of measuring the magnetic field of eddy currents that the instrument induces in the formation adjoining the borehole. The electrical conductivity is used for, among other reasons, inferring the fluid content of the earth formations. Typically, lower conductivity (higher resistivity) is associated with hydrocarbon-bearing earth formations. The physical principles of electromagnetic induction well logging are well described, for example, in, J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists, December 1962. Many improvements and modifications to electromagnetic induction resistivity instruments described in the Moran and Kunz reference, supra, have been devised, some of which are described, for example, in U.S. Pat. No. 4,837,517 to Barber, in U.S. Pat. No. 5,157,605 to Chandler et al., and in U.S. Pat. No. 5,600,246 to Fanini et al.
Conventional induction well logging techniques employ a metal pipe inside a coil mandrel. One or more transmitter coils are energized by an alternating current. The oscillating magnetic field produced by this arrangement results in the induction of currents in the formations which are nearly proportional to the conductivity of the formations. These currents, in turn, contribute to the voltage induced in one or more receiver coils. By selecting only the voltage component which is in phase with the transmitter current, a signal is obtained that is approximately proportional to the formation conductivity. In a conventional induction logging apparatus, the basic transmitter coil and receiver coil have axes which are aligned with the longitudinal axis of the well logging device. (For simplicity of explanation, it will be assumed that the borehole axis is aligned with the axis of the logging device, and that these are both in the vertical direction. Also single coils will subsequently be referred to without regard for focusing coils or the like.) This arrangement tends to induce secondary current loops in the formations that are concentric with the vertically oriented transmitting and receiving coils. The resultant conductivity measurements are indicative of the horizontal conductivity (or resistivity) of the surrounding formations. There are, however, various formations encountered in well logging which have a conductivity that is anisotropic. Anisotropy results from the manner in which formation beds were deposited by nature. For example, “uniaxial anisotropy” is characterized by a difference between the horizontal conductivity, in a plane parallel to the bedding plane, and the vertical conductivity, in a direction that is commonly perpendicular to the bedding plane. When there is no bedding dip, horizontal resistivity can be considered to be in the plane perpendicular to the borehole, and the vertical resistivity in the direction parallel to the borehole. Conventional induction logging devices, which tend to be sensitive only to the horizontal conductivity of the formations, do not provide a measure of vertical conductivity or of anisotropy. Techniques have been developed to determine formation anisotropy. See, e.g. U.S. Pat. No. 4,302,722 to Gianzero et al. Transverse anisotropy often occurs such that variations in resistivity occur in the azimuthal direction.
Multi-component signals can be used for interpreting formation resistivities and petrophysical parameters. The principles used for this interpretation have been discussed, for example, in U.S. Pat. No. 6,470,274 to Mollison et al., U.S. Pat. No. 6,643,589 to Zhang et al., U.S. Pat. No. 6,636,045 to Tabarovsky et al., the contents of which are incorporated herein by reference. Specifically, the parameters estimated may include horizontal and vertical resistivities (or conductivities), relative dip angles, strike angles, sand and shale content and water saturation. In addition, U.S. patent application Ser. No. 11/125,530 of Rabinovich et al. teaches the use of multi-component measurements for analysis of fractured earth formations that may also have anisotropic layers. These multi-component signals are typically obtained using a multi-component measurement tool having coils oriented transverse to the tool axis in addition to coils oriented parallel to the tool axis.
In addition to formation response, resistivity measurements can be affected by magnetic fields that arise from non-formation effects. Two such non-formation effects result from tool eccentricity within the borehole and coil misalignment with respect to the tool axis. Induction tools generally give rise to a current flow in the conductive drilling mud that surrounds the tool and fills the borehole. Tool eccentricity generally causes more problems to transverse (X or Y) coils than to axial (Z) coils. FIGS. 3A-C show cross-sectional views of an induction tool having a non-conductive mandrel at different positions within a borehole. FIG. 3A shows an x-oriented tool 304 that is centered within the borehole 302 filled with mud 306. In one aspect, the current induced in the borehole generally flows along the axial channel 307 and in the opposite direction along the axial channel 309. Due to the symmetry of the current flow channels (307 and 309), the centered induction tool does not experience an eccentricity effect. In FIG. 3B, the tool is decentralized along the x-direction. Due to the orientation of the transmitter, the current flowing along channels 307 and 309 still displays symmetry and thus this eccentricity generally does not affect the measurements much. FIG. 3C shows the induction tool decentralized along the y-axis, such that channel 307 is constricted while the lower channel is broadened 309. Additionally, current flowing in channel 307 may interact with the formation. Thus, the borehole current flow is highly affected due to decentralization along the y-axis. The net borehole current induces signals in transverse receiver coils, especially coplanar transmission and receiver coils. The net current may also induce signals in axial receiver coils that are at different axial positions from the transverse transmitter coil. Because the induction current density increases with increasing mud conductivity, the net current-induced signals are stronger for higher mud conductivity.
The eccentricity effects may be reduced by using a conductive mandrel. However, such a conductive mandrel is highly susceptible to the effects of coil misalignment. The coil misalignment effect is due to the position of coils with respect to the tool axis or inner pipe. Induction tool coils are typically disposed on a pipe which may be of a highly conductive metal. The pipe serves several purposes, such as protecting and shielding through-wires and supporting the tool weight. A transverse transmitter coil induces an induction current in the pipe if the pipe forms a closed loop with other paths of electric current. However, the pipe current does not necessarily distort the tool measurement. If the receiver coil, coplanar with the transmitter coil, is symmetric with respect to the pipe axis, the measurement is typically not affected. The pipe induction current affects the measurement only if the receiver coil also is asymmetric with respect to the pipe axis. An orthogonal transverse receiver coil may also be affected by coil misalignment. In addition to the coil misalignment error caused by the pipe eddy current, misaligned orthogonal coils will also induce direct coupling between the coils.
U.S. patent application Ser. No. 11/598,305 of Wang et al, having the same assignee as the present disclosure teaches the use of a conductive outer sleeve provided with a plurality of openings on the mandrel to reduce the effect of currents flowing in the borehole fluid. One drawback is that the sleeve is insulated from the metal pipe, so that the compensation may not be effective for all values of the current in the metal pipe. The present disclosure addresses this potential drawback.